1. Field of the invention
The present invention relates to methods of detecting defects formed in a skin layer of a honeycomb structural body having a structure in which a plurality of cells is formed by cell walls and the skin layer having a cylindrical shape covers the outer peripheral surface of the honeycomb structural body.
2. Description of the Related Art
Honeycomb structural bodies are widely known and used as supporting bodies which support catalysts capable of purifying exhaust gas emitted from internal combustion engines mounted to motor vehicles, etc. Such a honeycomb structural body is made of porous ceramics. In general, a honeycomb structural body has a plurality of cells and a skin layer of a cylindrical shape. The cells are surrounded by cell walls. The cell walls are arranged to form the cells so that each of the cells has a polygonal shape and is uniformly arranged when observed from a cross section of the honeycomb structural body which is perpendicular to an axial direction of the honeycomb structural body, i.e. a flow direction of exhaust gas. The honeycomb structural body having the structure previously described is mounted on the inside of an exhaust gas pipe through which exhaust gas emitted from an internal combustion engine flows. Because the exhaust gas emitted from an internal combustion engine has a high temperature, when the exhaust gas is supplied to the inside of the honeycomb structural body, the exhaust gas activates catalysts supported in the honeycomb structural body. As a result, catalyst reaction occurs and this purifies the exhaust gas. The purified exhaust gas is discharged to the outside of the exhaust gas pipe.
It is strongly requested for a honeycomb structural body to have no defects such as holes. There has been developed and widely used various detection methods of detecting presence of defects generated in a honeycomb structural body. For example, there is a conventional detection method of defecting defects generated in a ceramic filter such as a honeycomb structural body capable of capturing fine particles contained in exhaust gas emitted from a diesel engine. The ceramic filter is burned to eliminate the captured fine particles. This makes it possible to enable repetitive use of the ceramic filter. The detection method supplies compressed air to an inlet side of the ceramic filter. One or more acoustic sensors detect sound of leakage of the air at an outlet side of the ceramic filter in order to detect the presence of defects formed in the ceramic filter. A patent document 1, Japanese unexamined patent application publication (translation of PCT application) No. 2005-530957 discloses such a conventional detection method.
However, the conventional detection method previously described cannot detect defects generated in a skin layer of the ceramic filter as a honeycomb structural body. When defects such as cracks and holes are generated in the skin layer of the ceramic filter, catalysts supported in the inside of the ceramic filter are pushed and supplied as leaking catalysts to outside through the defects formed in the skin layer during a catalyst supporting process to support catalysts on the inside of the ceramic filter. This allows an unnecessary amount of catalysts to be consumed on the outer peripheral surface of the honeycomb structural body. Because catalysts are an expensive noble metal, this increases the manufacturing cost of the honeycomb structural body. In addition to this drawback, this causes the honeycomb structural body to have a poor appearance.